The present invention relates to photomasks, and more particularly, to an alternative phase shift mask (APSM) having a damascene structure formed for use with extreme ultraviolet lithography (EUVL).
Photolithography is a common step used in the manufacture of integrated circuits. In photolithography, a photomask is placed above the wafer. The photomask (also known as a reticle) contains the pattern that is to be replicated onto the wafer. Illuminating radiation is then projected onto the photomask.
In the case of a transmissive photomask, the mask pattern is created by transmissive portions and absorbing portions arranged in the pattern on the mask. A selected wavelength, for example, 248 nanometers (nm), of irradiating radiation is shined through the mask. The transmissive portions of the mask, which are transparent to the selective wavelength, allow the light to pass through the mask. The absorbing portions, which are opaque to and absorb the selected wavelength, block the transmission. The pattern on the mask is thereby replicated onto the photoresist on the device wafer.
In another type of photomask, known as a reflective mask, the photomask surface contains reflective portions and absorbing portions. When light of a selected wavelength is applied to the photomask, the light is reflected off the reflecting portions. The reflected image from the mask usually is further reflected off of a mirror or lens system, then onto the wafer.
Reflective photomasks are used when the illuminating radiation is in the EUV range. Patterning of the transmission mask using deep UV radiation, such as 193 nm wavelength, and vacuum UW radiation, such as 157 nm wavelength, are all currently being developed. Because EUV radiation is strongly absorbed by condensed matter, such as quartz, a reflective photomask is commonly used for EUVL.
Another method of increasing the resolution of a photolithography system is to combine alternative phase shift mask (APSM) technology with a EUVL reflective photomask. In this method, selected portions of a photomask are manufactured to introduce a 180 degree phase shift in the reflected light. Thus, the reflected light from the phase shifted portions of the photomask will destructively interfere with the reflected light from the non-phase shifted portions. This destructive interference intensity pattern can be used to pattern the photoresist on a wafer. This technology is described in U.S. Pat. No. 5,328,784 to Fukuda and in xe2x80x9cOptical Technology for EUV Lithographyxe2x80x9d by Ito et al., Optical Society of America, TOPS on Extreme Ultraviolet Lithography, Vol. 4 (1996).
In the prior art reflective APSM, referring to FIG. 1, the APSM 101 includes a substrate 103 that has various layers formed thereon. First, a phase shifting pattern 105 is deposited onto the substrate 103. The phase shifting pattern has a thickness of approximately xc2xc of the illuminating radiation wavelength, i.e., xcex/4. Next, a multilayer stack 107 comprising alternating thin film layers of molybdenum (Mo) and silicon (Si) is deposited. Typically, the multilayer stack 103 consists of 40 pairs of Mo/Si thin films, each pair of thin films approximately 7 nm in thickness. The multilayer stack 103 will reflect EUV radiation. Formed atop of the multilayer stack 103 is a patterned absorptive metal layer 109. The patterned absorptive metal layer 109 covers the transitions between areas of the substrate 103 that have the phase shift patterns 105 and those that do not. By varying the widths of the absorptive metal layer 109, features having different sizes can be patterned.
This prior art photomask has some disadvantages. First, the photomask 101 of FIG. 1 introduces a shadowing effect. In EUVL, the incident radiation comes at an angle from normal due to the nature of a reflective mask. The combination of oblique illumination with a non-zero height of the absorptive metal layer 109 causes a shadowing effect, which needs to be corrected by adjusting the size of the photomask features. Typically, the photomask is biased toward a smaller dimension in order to compensate for the shadowing effect. As EUVL technology extends to smaller design rules, the biasing requirement may place a limitation on EUVL mask fabrication. Further, the prior art photomask 101 is not planar, leading to possible damage during cleaning of the surface of the photomask. Other disadvantages of the prior art photomask 101 will become apparent as the detailed description of the present invention is reviewed.